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Batch Reactor
Optical High Precision Components by Hellma in the European Columbus Space Laboratory
The batch reactor is used for researching crystal growth in weightless conditions. In this environment, the
structure and function of biological macromolecules (Proteins) can be investigated particularly well. In order
to guarantee perfect measurements the batch reactor is manufactured using synthetic quartz glass. High-
grade polished components were connected to a monolithic quartz component using thermal bonding
technology without using additional components. The demands made on technology, precision and
tolerances are extremely high. In order to improve signal yield during the measuring process the surfaces
were high-quality and nonreflective. The results of the experiments are directly channelled into biological
and medical research to improve understanding of protein synthesis as well as gaining an important insight
into fighting infections and other illnesses. The batch reactors form part of a so-called Protein Crystallisation
Diagnostics Facility (PCDF) and were taken on 7th February 2008 to the International Space Station ISS on
the 24th launch of the NASA Space Shuttle Atlantis.
Batch Reactors
Type of Reactor Characteristics
Simple Batch Reactor is charged via two holes in the top of the tank; while reaction is carried out, nothing else is put in or taken out until the reaction is done; tank easily heated or cooled by jacket
Kinds of Phases Present
Usage Advantages Disadvantages
1. Gas phase
2. Liquid phase
3. Liquid Solid
1. Small scale production
2. Intermediate or one shot production
3. Pharmaceutical
4. Fermentation
1. High conversion per unit volume for one pass
2. Flexibility of operation-same reactor can produce one product one time and a different product the
1. High operating cost
2. Product quality more variable than with continuous operation
next
3. Easy to clean
General Mole Balance Equation
Assumptions
1) No inflow or out flow FA0 = FA = 0
2) Well mixed
Constant volume V = V0 (e.g. Liquid Phase or Gas Phase in a steel container)
In terms of conversion
The reaction time necessary to reach a conversions X in a batch reactor is
. The following table gives reaction times for first (–rA = kCA) and second (–
rA = k ) in a batch reactor
The following table gives the various times necessary to process one complete batch.
AUXILLIARIES
The Batch reactor is the generic term for a type of vessel widely used in the process industries. Its name
is something of a misnomer since vessels of this type are used for a variety of process operations such
as solids dissolution, product mixing, chemical reactions, batch distillation, crystallization, liquid/liquid
extraction andpolymerization. In some cases, they are not referred to as reactors but have a name which
reflects the role they perform (such as crystallizer, or bio reactor).
A typical batch reactor consists of a tank with an agitator and integral heating/cooling system. These
vessels may vary in size from less than 1 litre to more than 15,000 litres. They are usually fabricated
in steel, stainless steel, glass lined steel, glass or exotic alloy. Liquids and solids are usually charged via
connections in the top cover of the reactor. Vapors and gases also discharge through connections in the
top. Liquids are usually discharged out of the bottom.
The advantages of the batch reactor lie with its versatility. A single vessel can carry out a sequence of
different operations without the need to break containment. This is particularly useful when
processing, toxic or highly potent compounds.
Plug Flow Reactor
The plug flow reactor (PFR) model is used to describe chemical reactions in continuous, flowing
systems. The PFR model is used to predict the behaviour of chemical reactors, so that key reactor
variables, such as the dimensions of the reactor, can be estimated. PFR's are also sometimes called
Continuous Tubular Reactors (CTR's).
Schematic diagram of a Plug Flow Reactor (PFR)
Fluid going through a PFR may be modeled as flowing through the reactor as a series of infinitely thin
coherent "plugs", each with a uniform composition, traveling in the axial direction of the reactor, with each
plug having a different composition from the ones before and after it. The key assumption is that as a plug
flows through a PFR, the fluid is perfectly mixed in the radial direction but not in the axial direction
(forwards or backwards). Each plug of differential volume is considered as a separate entity, effectively an
infinitesimally small batch reactor, limiting to zero volume. As it flows down the tubular PFR, the residence time (τ) of the plug is a function of its position in the reactor. In the ideal PFR, the residence time
distribution is therefore a Dirac delta function with a value equal to τ.
PFR modeling
PFR are frequently referred to as piston flow reactors, or sometimes as continuous tubular reactors. They
are governed by ordinary differential equations, the solution for which can be calculated providing that
appropriate boundary conditions are known.
The PFR model works well for many fluids: liquids, gases, and slurries. Although turbulent flow and axial
diffusion cause a degree of mixing in the axial direction in real reactors, the PFR model is appropriate
when these effects are sufficiently small that they can be ignored.
In the simplest case of a PFR model, several key assumptions must be made in order to simplify the
problem, some of which are outlined below. Note that not all of these assumptions are necessary,
however the removal of these assumptions does increase the complexity of the problem. The PFR model
can be used to model multiple reactions as well as reactions involving changing temperatures, pressures
and densities of the flow. Although these complications are ignored in what follows, they are often
relevant to industrial processes.
Assumptions:
plug flow
steady state
constant density (reasonable for some liquids but a 20% error for polymerizations; valid for gases
only if there is no pressure drop, no net change in the number of moles, nor any large temperature
change)
single reaction
A material balance on the differential volume of a fluid element, or plug, on species i of axial
length dx between x and x + dx gives
[accumulation] = [in] - [out] + [generation] - [consumption]
1. Fi(x) − Fi(x + dx) + Atdxνir = 0 . [1]
When linear velocity, u, and molar flow rate relationships, Fi, and ,
are applied to Equation 1 the mass balance on i becomes
2. . [1]
When like terms are canceled and the limit dx → 0 is applied to Equation 2 the mass balance on
species i becomes
3. , [1]
where Ci(x) is the molar concentration of species i at position x, At the cross-sectional area of the tubular reactor, dx the differential thickness of fluid plug, and νi stoichiometric coefficient. The reaction rate, r, can
be figured by using the Arrhenius temperature dependence. Generally, as the temperature increases so does the rate at which the reaction occurs. Residence time, τ, is the average amount of time a discrete
quantity of reagent spends inside the tank.
Assume:
isothermal conditions, or constant temperature (k is constant)
single, irreversible reaction (νA = -1)
first-order reaction (r = kCA)
After integration of Equation 3 using the above assumptions, solving for CA(L) we get an explicit equation
for the output concentration of species A,
4. ,
where CAo is the inlet concentration of species A.
[edit]Operation and uses
PFRs are used to model the chemical transformation of compounds as they are transported in systems
resembling "pipes". The "pipe" can represent a variety of engineered or natural conduits through which
liquids or gases flow. (e.g. rivers, pipelines, regions between two mountains, etc.)
An ideal plug flow reactor has a fixed residence time: Any fluid (plug) that enters the reactor at time t will
exit the reactor at time t + τ, where τ is the residence time of the reactor. The residence time distribution
function is therefore a dirac delta function at τ. A real plug flow reactor has a residence time distribution
that is a narrow pulse around the mean residence time distribution.
A typical plug flow reactor could be a tube packed with some solid material (frequently a catalyst).
Typically these types of reactors are called packed bed reactors or PBR's. Sometimes the tube will be a
tube in a shell and tube heat exchanger.
[edit]Advantages and disadvantages
CSTRs (Continuous Stirred Tank Reactor) and PFRs have fundamentally different equations, so the
kinetics of the reaction being undertaken will to some extent determine which system should be used.
However there are a few general comments that can be made with regards to PFRs compared to other
reactor types.
Plug flow reactors have a high volumetric unit conversion, run for long periods of time without
maintenance, and the heat transfer rate can be optimized by using more, thinner tubes or fewer, thicker
tubes in parallel. Disadvantages of plug flow reactors are that temperatures are hard to control and can
result in undesirable temperature gradients. PFR maintenance is also more expensive than CSTR
maintenance. [2]
Through a recycle loop a PFR is able to approximate a CSTR in operation. This occurs due to a decrease
in the concentration change due to the smaller fraction of the flow determined by the feed; in the limiting
case of total recycling, infinite recycle ratio, the PFR perfectly mimics a CSTR.
Fluidized bed reactorA fluidized bed reactor (FBR) is a type of reactor device that can be used to carry out a variety
of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a granular solid
material (usually a catalyst possibly shaped as tiny spheres) at high enough velocities to suspend the solid and
cause it to behave as though it were a fluid. This process, known as fluidization, imparts many important
advantages to the FBR. As a result, the fluidized bed reactor is now used in many industrial applications.
Basic principles
The solid substrate (the catalytic material upon which chemical species react) material in the fluidized bed
reactor is typically supported by a porous plate, known as a distributor.[1] The fluid is then forced through
the distributor up through the solid material. At lower fluid velocities, the solids remain in place as the fluid
passes through the voids in the material. This is known as a packed bed reactor. As the fluid velocity is
increased, the reactor will reach a stage where the force of the fluid on the solids is enough to balance the
weight of the solid material. This stage is known as incipient fluidization and occurs at this minimum
fluidization velocity. Once this minimum velocity is surpassed, the contents of the reactor bed begin to
expand and swirl around much like an agitated tank or boiling pot of water. The reactor is now a fluidized
bed. Depending on the operating conditions and properties of solid phase various flow regimes can be
observed in this reactor.
[edit]History and current uses
Fluidized bed reactors are a relatively new tool in the chemical engineering field. The first fluidized bed
gas generator was developed by Fritz Winkler in Germany in the 1920s.[2] One of the first United States
fluidized bed reactors used in the petroleum industry was the Catalytic Cracking Unit, created in Baton
Rouge, LA in 1942 by the Standard Oil Company of New Jersey (nowExxonMobil).[3] This FBR and the
many to follow were developed for the oil and petrochemical industries. Here catalysts were used to
reduce petroleum to simpler compounds through a process known as cracking. The invention of this
technology made it possible to significantly increase the production of various fuels in the United
States. [4] In the late 1980's, the work of Gordana V. Novakovic, Robert S. Langer, V.A. Shiva
Ayyadurai and others began the use of fluidized bed reactors in biological sciences for understanding and
visualizing the fluid dynamics of blood deheparinization.[5][6]
Today fluidized bed reactors are still used to produce gasoline and other fuels, along with many other
chemicals. Many industrially produced polymers are made using FBR technology, such asrubber, vinyl
chloride, polyethylene, and styrenes. Various utilities also use FBR’s for coal gasification, nuclear power
plants, and water and waste treatment settings. Used in these applications, fluidized bed reactors allow
for a cleaner, more efficient process than previous standard reactor technologies.[4]
[edit]Advantages
The increase in fluidized bed reactor use in today’s industrial world is largely due to the inherent
advantages of the technology.[7]
Uniform Particle Mixing: Due to the intrinsic fluid-like behavior of the solid material, fluidized
beds do not experience poor mixing as in packed beds. This complete mixing allows for a uniform
product that can often be hard to achieve in other reactor designs. The elimination of radial and axial
concentration gradients also allows for better fluid-solid contact, which is essential for reaction
efficiency and quality.
Uniform Temperature Gradients: Many chemical reactions require the addition or removal of
heat. Local hot or cold spots within the reaction bed, often a problem in packed beds, are avoided in a
fluidized situation such as an FBR. In other reactor types, these local temperature differences,
especially hotspots, can result in product degradation. Thus FBRs are well suited
to exothermic reactions. Researchers have also learned that the bed-to-surface heat
transfer coefficients for FBRs are high.
Ability to Operate Reactor in Continuous State: The fluidized bed nature of these reactors
allows for the ability to continuously withdraw product and introduce new reactants into the reaction
vessel. Operating at a continuous process state allows manufacturers to produce their various
products more efficiently due to the removal of startup conditions in batch processes.
[edit]Disadvantages
As in any design, the fluidized bed reactor does have it draw-backs, which any reactor designer must take
into consideration.[7]
Increased Reactor Vessel Size: Because of the expansion of the bed materials in the reactor, a
larger vessel is often required than that for a packed bed reactor. This larger vessel means that more
must be spent on initial capital costs.
Pumping Requirements and Pressure Drop: The requirement for the fluid to suspend the solid
material necessitates that a higher fluid velocity is attained in the reactor. In order to achieve this,
more pumping power and thus higher energy costs are needed. In addition, the pressure
drop associated with deep beds also requires additional pumping power.
Particle Entrainment: The high gas velocities present in this style of reactor often result in fine
particles becoming entrained in the fluid. These captured particles are then carried out of the reactor
with the fluid, where they must be separated. This can be a very difficult and expensive problem to
address depending on the design and function of the reactor. This may often continue to be a
problem even with other entrainment reducing technologies.
Lack of Current Understanding: Current understanding of the actual behavior of the materials
in a fluidized bed is rather limited. It is very difficult to predict and calculate the complex mass and
heat flows within the bed. Due to this lack of understanding, a pilot plant for new processes is
required. Even with pilot plants, the scale-up can be very difficult and may not reflect what was
experienced in the pilot trial.
Erosion of Internal Components: The fluid-like behavior of the fine solid particles within the bed
eventually results in the wear of the reactor vessel. This can require expensive maintenance and
upkeep for the reaction vessel and pipes.
Pressure Loss Scenarios: If fluidization pressure is suddenly lost, the surface area of the bed
may be suddenly reduced, this can either be an inconvenience (e.g. making bed restart difficult), or
may have more serious implications, such as runaway reactions (e.g. for exothermic reactions in
which heat transfer is suddenly restricted).
[edit]Current research and trends
Due to the advantages of fluidized bed reactors, a large amount of research is devoted to this technology.
Most current research aims to quantify and explain the behavior of the phase interactions in the bed.
Specific research topics include particle size distributions, various transfer coefficients, phase
interactions, velocity and pressure effects, and computer modeling.[8] The aim of this research is to
produce more accurate models of the inner movements and phenomena of the bed. This will enable
chemical engineers to design better, more efficient reactors that may effectively deal with the current
disadvantages of the technology and expand the range of FBR use.
Fluidized-Bed Reactor
A fluidized-bed reactor is a combination of the two most common, packed-bed and stirred tank, continuous flow reactors. It is very important to chemical engineering because of its excellent heat and mass transfer characteristics. The fluidized-bed reactor can be seen below:
In a fluidized-bed reactor, the substrate is passed upward through the immobilized enzyme bed at a high enough velocity to lift the particles. However, the velocity must not be so high that the enzymes are swept away from the reactor entirely. This causes some mixing, more than the piston-flow model in the packed-bed reactor, but complete mixing as in the CSTR model. This type of reactor is ideal for highly exothermic reactions because it eliminates local hot-spots, due to its mass and heat transfer characteristics mentioned before. It is most often applied in immobilized-enzyme catalysis where viscous, particulate substrates are to be handled.
Semibatch Reactors
Semibatch Reactors
The reactant that starts in the reactor is always the limiting reactant.
Three Forms of the Mole Balance Applied to Semibatch Reactors:
1. Molar Basis
2. Concentration Basis
3. Conversion
For constant molar feed:
For constant density:
Use the algorithm to solve the remainder of the problem.
Example: Elementary Irreversible Reaction
Consider the following irreversible reaction:
The combined mole balance, rate law, and stoichiometry may be written in terms of conversion and/or concentration:
Conversion Concentration Number of Moles
Polymath Equations:
Conversion Concentration Molesd(X)/d(t) = -ra*V/Nao d(Ca)/d(t) = ra - (Ca*vo)/V d(Na)/d(t) = ra*V
ra = -k*Ca*Cbd(Cb)/d(t) = rb + ((Cbo-Cb)*vo)/V
d(Nb)/d(t) = rb*V + Fbo
Ca = Nao*(1 - X)/V ra = -k*Ca*Cb ra = -k*Ca*Cb
Cb = (Nbi + Fbo*t - Nao*X)/V
rb = ra rb = ra
V = Vo + vo*t V = Vo + vo*t V = Vo + vo*tVo = 100 Vo = 100 Vo = 100vo = 2 vo = 2 vo = 2Nao = 100 Fbo = 5 Fbo = 5Fbo = 5 Nao = 100 Ca = Na/VNbi = 0 Cbo = Fbo/vo Cb = Nb/Vk = 0.1 k = 0.01 k = 0.01
Na = Ca*V
X = (Nao-Na)/Nao
Semi batch reactors or fed batch reactors
The semi batch reactor is similar to the batch reactor but has the additional feature of
continuous addition or removal of one or more components / streams. In addition to better
yields and selectivity, gradual addition or removal assists in controlling temperature
particularly when the net reaction is highly exothermic. Thus, use of a semi batch reactor
intrinsically permits more stable and safer operation than in a batch operation. Fed batch
reactors are rarely used in waste water treatment units.